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  • Electrical safety in anaesthesia and surgery

     

    JOHN T. B. MOYLE

     

     

    The subject of electrical safety in the operating theatre and elsewhere in health-care facilities, is of the utmost importance for the following reasons:

     

    (1)electronic equipment is now ubiquitous in health care;

    (2)in no other sphere of life is the deliberate ohmic connection of electronic equipment to living tissue entertained; and

    (3)the sick patient is more susceptible to the unwanted effects of electrical energy than is the healthy person.

     

    HARMFUL EFFECTS OF CURRENT ELECTRICITY

    There are a number of ways by which an electric current passing through living tissue may cause damage. With an understanding of these mechanisms, electrical safety enters the realm of common sense.

     

    Risk of electrical damage may occur whenever the body, or part of it, becomes part of an electrical circuit. The amount and variety of morbidity and mortality depend upon the magnitude of the current, the time for which it passes through the body, and, to a certain extent, the frequency of the applied current.

     

    From Ohm's law, I = E/ Z where I is the current in amperes, E = the potential difference in volts across Z, which is the resistance (or impedance with alternating current (a.c.)). The current passing through the body therefore depends not only upon the magnitude of the applied voltage but also upon the electrical resistance of the body; the lower the resistance, the higher the current. If the skin was perfectly dry, the skin resistance alone would be between 100 000 ohm and 300 000 ohm. However, water and, especially, perspiration reduce this resistance dramatically, such that the average resistance may be assumed to be between 50 ohms and 10 000 ohm when assessing electrical risk. Table 1 34 shows the approximate resistivity of body tissues at 50 Hz.

     

    The morbidity of excessive currents passing through the living body may be due to one or more of the following:

     

    (1)electrical energy being converted into heat which will cause damage proportional to the product of time and current; this may even progress to charring (a process made use of in radio frequency surgical diathermy);

    (2)hypoxaemic damage due to respiratory muscle spasm, or temporary cardiac arrhythmia; permanent cardiac damage may also ensue;

    (3)chemical burns at contact points due to electrolysis—this type of morbidity only occurs when there is a direct current (d.c.) component to the electric current.

     

    Death from electrocution may occur due to asphyxia caused by the respiratory muscle spasm, respiratory arrest due to CNS dysfunction, or because of cardiac asystole or arrhythmia.

     

    Damage to any particular part of the body is dependent upon current density or current per unit cross-sectional area of the current pathway. This concept is illustrated in Fig. 1 47.

     

    Table 2 35 shows the effects of increasing current at 50 or 60 Hz through the human trunk. These values vary greatly under different conditions, the most obvious of which is the route that the current takes through the body—the most dangerous of which is that which passes through the axis of the heart. Other factors causing variation include sex, body weight, and state of health of the area of contact, and the frequency (see Chapter 3.2 5), the waveform, and the duration of the electric shock.

    Microshock

    The concept of microshock is important in anaesthesia, intensive care, and thoracic surgery. If electrical contact is made internally, especially on or close to the heart, very low currents, as low as 10 &mgr;A, may initiate arrhythmias. The resistance of skin contact is eliminated and the current density at the interface between the contact and the heart is very high (see Fig. 3 49). Microshock may occur during thoracic surgery, but the risk is much more common when conductive saline is used as the fluid in a cardiac catheter, pulmonary artery catheter, or a central venous pressure catheter, as leakage current may pass through a pressure transducer.

     

    PROTECTION AGAINST ELECTRICAL INJURIES AND ELECTROCUTION

    The philosophy of electrical safety in medicine has a different bias from that in the domestic or industrial situation. The reasons for this are:

     

    1.Only in the medical environment is direct electrical connection with the body necessary (surgical diathermy, electrocardiogram, electromyogram, electroencephalogram, etc.);

    2.Any protective cut-out system must be sensitive enough to protect against currents above preset, but very low, values due to fault conditions, passing through the body. Such sensitive devices are very likely to trip-out in non-fault conditions; this would also be dangerous to life, in the case of ventilators, dialysis machines, and extracorporeal circulation pumps.

    3.Protective cut-outs would have to be built into each piece of electronic apparatus and designed in such a way that a fault condition occurring in one piece of equipment did not cause the power supply of other life-supporting equipment to be tripped-out.

     

    Electrical safety in the domestic and industrial environment and for the doctor's surgery (office), where single items of diagnostic apparatus which are not life supporting and conventional office equipment are used, is nowadays provided by a device called an earth leakage circuit breaker (ELCB). The principle of the ELCB is shown in Fig. 4 50. The live and neutral conductors are passed through a ferrite ring, thus forming the single turn primary of a transformer. The secondary winding consists of many turns of fine-gauge wire, which are connected to a solenoid. Under normal conditions, with no fault in the apparatus supplied through the ELCB, the current in the neutral conductor is equal and opposite in polarity to that in the live conductor, and therefore there is no change in the magnetic field and no current is induced to energize the solenoid.

     

    If a fault occurs such that electrical leakage current passes out of the system, say, through a human body, then there will be an imbalance of current between the live and neutral conductors and a voltage will be induced to energize the solenoid. Energization of the solenoid mechanically disconnects the live and neutral conductors from the apparatus. The ELCB has to be manually reset after the fault condition has been rectified. ELCBs are normally arranged to supply more than one socket outlet, which means that a single fault may disconnect the supply from a number of pieces of equipment.

     

    PREVENTION OF GROSS ELECTROCUTION IN THE OPERATING THEATRE

    The ELCB is unsuitable for use in the operating theatre and intensive care unit for the following reasons.

     

    1Permissible levels of electric current that may be allowed to pass through the sick, and so electrically susceptible, patient are much lower than those permissible in the fit and healthy person.

    2If an ELCB is designed to be extra-sensitive, it is always more prone to erroneous cut-outs.

    3An electrical current may be deliberately allowed to pass through the patient and may not all return to the equipment in use, but may find an alternative pathway. This may be at extremely low levels in monitoring equipment, but at increasing levels through (muscle or nerve) stimulators, to appreciable values with surgical diathermy.

     

    Because of these problems, the idea in medical electronics is to use ever more rigorous design safety criteria, and by patient–circuit isolation rather than the use of ELCBs.

     

    All equipment that may come into contact with a patient should have been designed and manufactured to comply with the relevant national and international standards. International agreement about safe design is published by the International Electrotechnical Commission (IEC) in the Standard IEC 601 (= BS 5724). This standard is published as IEC 601 Part 1, which contains all the general requirements for all equipment. There are an increasing number of Part 2s being published, which contain particular requirements for each type of equipment.

     

    The main protection against electrocution in the operating theatre and elsewhere in the health-care facility is insulation, which minimizes the leakage current that may pass through the human body. Insulation, which also refers to the protective housing of equipment, must be designed so that possible leakage currents do not exceed levels set out in Standard IEC 601 even if the earth connection is inadequate. Earth conductors at mains supply outlets should be tested regularly, and the external cables of equipment should also be inspected carefully on a regular basis. These tests should be carried out by qualified engineering staff. However, electrical safety is also the responsibility of the users of the equipment.

     

    1.Avoid portable distribution boards whenever possible.

    2.Use ceiling-mounted pendant supplies whenever possible, as they are less likely to be damaged than those on the floor and are unlikely to become wet. Keep water and electricity apart.

    3.Avoid the use of long mains supply cables, and avoid damage to cables by knotting, equipment wheels, etc.

    4.Notify engineering staff of any visible damage to equipment or cables.

    5.Make sure that regular maintenance records are kept and are available for inspection by the user.

     

    One area where a potential risk of electric shock or burns is not common sense, except to an engineer, is when high frequency currents are in use, for example with radio frequency surgical diathermy. In normal use, insulation between two conductors or a single conductor and the body may be entirely adequate for d.c. or 50 Hz. However, this insulation may become the dielectric of a capacitor formed between the body and a ‘conductor’ at high frequency. (The resistance or impedance of a capacitor decreases as frequency increases.) Thus, at high frequencies, high currents may pass along unexpected routes, causing electrocution or burns. Burns may even occur between the body and metalwork that is not intentionally a conductor but is earthed. Burns due to this mechanism have occurred via the metalwork of operating tables and through the transducers of pulse oximeters (modern pulse oximeter transducers are isolated so that an earth pathway at high frequency cannot occur).

     

    PREVENTION OF MICROSHOCK

    Conventional safety measures may not protect the ‘electrically susceptible’ patient sufficiently. IEC 601 classifies the extra requirements in the design of equipment where there is risk of microshock.

     

    Category BFEquipment having an applied part with intentional connection to the patient (e.g. electrocardiogram, electroencephalogram, electromyogram).

    Category CFEquipment with points specifically designed for application where a conductive connection directly to the heart is established.

     

    In general, the maximum patient leakage current with BF equipment is 100 &mgr;A under normal conditions and 500 &mgr;A with a single fault condition. With CF equipment, the maximum currents are 10 &mgr;A and 50 &mgr;A, respectively. If CF equipment is connected to a patient, all other devices connected to the patient at that time should be CF rated, otherwise leakage currents may find other routes to the heart.

     

    In this day and age of microcomputers, care must be taken when computing equipment is connected in any way to monitoring apparatus, as most computing equipment is not protected to the same high specification as medical equipment and the safety standard of such monitoring equipment is immediately lowered, by definition, to that of the computing equipment. Some form of electrical isolation should be interposed between the medical device and the computer.

     

    The subject of electrical safety in the health-care situation is complex. Safety is only possible through good maintenance, vigilance, and common sense on the part of the user, and by only using equipment that conforms to IEC 601. However well these guidelines are followed, one must remember that nothing is absolutely foolproof.

     

    FURTHER READING

    Geddes LA, Baker LE. The specific resistance of biological material—A compendium of data for the biological engineer and physiologist. Med Bio Engineer 1967; 5: 271–93.

    IEC 479–1 Effects of passing current through the human body. Geneva: International Electrotechnical Commission, 1984.

    IEC 601–1 Safety of medical electrical equipment, Part 1: General requirements. Geneva: International Electrotechnical Commission, 1979.

    Martin TL. Malice in Blunderland. New York: McGraw Hill, 1973; 5.



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